Faulted circuit indicators (FCIs) are well known in the field of electric power distribution systems. Generally, FCIs are electrically connected to transmission lines in a power distribution system at various locations throughout the system, often in close proximity to system loads. When a fault occurs in a transmission line, FCIs between the power source and the fault will detect that a fault has occurred. Typically, FCIs that have detected a fault then display an indication that the fault has been detected. An FCI also can include a conventional transmitter for communicating faults to another location. A technician can then identify a fault by locating the transmission line between an FCI that indicates it has detected a fault and an FCI that does not indicate a fault.
FCIs and other types of sensors may be deployed widely to monitor systems in remote locations and in harsh environmental conditions. The difficulty of powering remote sensors, such as FCIs, has long been a problem limiting the sensors' lifespan and usefulness. Generating adequate power to a remote sensor requires the use of large primary batteries, which is cost prohibitive. As a result of a lack of sufficient power to remote sensors, the sensors must power down for periods of time to conserve energy, which results in a lack of communication from the sensors during down time.
Lithium primary cells, which are not rechargeable, provide one option as a power source for remote sensors. However, most transmitters for remote sensors require instantaneous power in amounts too large for lithium primary cells to provide. Additionally, non-rechargeable batteries require routine replacement at relatively short intervals.
Remote sensors could harvest power from other sources, such as solar, inductive, biological, or mechanical means. However, the inconsistent nature of these sources requires a system to capture the energy and store it for later use. Additionally, conventional sensors cannot withstand the harsh environments—−45 to +85 degrees Celsius—in which remote sensors usually are deployed can pose nearly insurmountable challenges, especially in the small packages required by modern electronics. Obtaining a life expectancy of more than two to three years for conventional sensors is unlikely in the absence of burdensome extra circuitry and devices that cool or heat the sensor to keep the charge receptor at or close to room temperature.
Several conventional devices have been developed to try to address the deficiencies associated with conventional sensors and their power sources. However, all such conventional devices have shortcomings. The various faults of these devices result in remote sensors that are expensive, short lived, or both. For example, a conventional solution is to use devices such as supercapacitors to power a remote sensor. However, supercapacitors work well only in applications that operate in environments close to 25 degrees Celsius. Thus, supercapacitors fail to provide the necessary functionality in the extreme environments of remote sensing applications.
In addition, lithium polymer and lithium ion cells have been identified as options. However, charging these cells can be difficult because of the environmental constraints of remote sensing. Conventionally, such cells are charged to their maximum allowable voltage, which yields the maximum capacity. Storing lithium cells when charged to maximum capacity will severely limit the cells' life expectancy. Fully charging a rechargeable lithium cell greatly degrades the cell, causing premature failure and reducing shelf life to less than three to four years. Additionally, developing circuitry to monitor and charge these systems at the available low power inputs can be prohibitive.
As discussed previously, lithium cells ordinarily are charged to the maximum allowable voltage. By charging to the maximum voltage, users of laptop computers, for example, can extend the maximum period of continuous use. However, this method of charging results in successive periods of charging to the maximum voltage capacity followed by periods of partially discharging the lithium cell. Such patterns of use severely degrade the life of a battery. Accordingly, traditional methods of charging lithium cells cannot provide the consistency and longevity required in remote sensing applications.
Accordingly, a need exists in the art for a power source for remote sensors that addresses the deficiencies of conventional sensors and associated power sources. For example, a need exists in the art for extended battery life in remote sensing operations. A further need exists for a circuit for float charging a battery to a predetermined voltage that will extend the life of the battery. A need also exists for float charging a battery as a way to provide sufficient and consistent power to remote sensors and to prevent down time resulting in loss of communication.